The present invention relates to a device and to a method for limiting or arresting the propagation of a buckle in a double-walled (or double-sheath) pipe consisting of two, respectively inner and outer, coaxial tubes separated by an annular space, this pipe being a rigid pipe for transporting fluids such as hydrocarbons, and designed to be submerged.
A rigid pipe or tube is laid on the seabed usually from a vessel called a pipelaying barge. The laying is called S-laying when the pipe adopts the shape of an S between the pipelaying barge and the seabed and it is called J-laying when the pipe adopts the shape of a J. In the latter case, a guide ramp is provided on the pipelaying barge, which ramp may sometimes be partially immersed in the water.
The rigid pipe to be laid is stored on the pipelaying barge either in pipe sections of a given but relatively short length, the pipe sections being joined together as the laying progresses, or it is made as a very long pipe on land and then wound onto a storage reel located on the pipelaying barge, the pipe then being unwound from the said reel during the laying operation. These laying operations are described in the API (American Petroleum Institute) document “Recommended Practice 17 A” from 1987.
When the pipe has left the barge and while the said pipe is being laid, it is important for the latter not to undergo plastic deformation in bending, which would result in ovalization of the pipe, the said ovalization causing a “weak singularity” which would be conducive to the initiation of a collapse. Moreover, when the pipe is laid on the seabed at great water depths (typically greater than 300 m and possibly down to 2000 m and more), the hydrostatic pressure exerted on the pipe may be sufficient to initiate a radial buckle which has a tendency to propagate along the pipe, in both directions. Of course, the buckle will form preferentially at a “weak singularity” when one exists on the pipe. When the buckle occurs, it is then necessary to replace at least that section or portion of the pipe comprising the buckled or collapsed region. The buckle propagation pressure is given by the formula:A×σ0×(T/D)βwhere σ0 is the yield stress of the steel, T is the thickness of the pipe and D is the external diameter of the pipe and where the parameters A and β are given by the American recommendations API RP 1111 (namely, A=24, β=2.4) or Norwegian recommendations DNV OS/F100 (namely, A=26.7, β=2.5). To resist the propagation of a buckle, the corresponding pressure must be greater than the hydrostatic pressure.
To avoid this problem, it is possible to increase the thickness of the pipe so as to have a propagation pressure greater than the hydrostatic pressure, but this results in a considerable increase in both the cost and the weight of the pipe, most particularly in the case of double-walled rigid pipes.
To prevent the propagation of a local buckle or local buckles, it has also been proposed to provide the pipe with certain devices or means, called buckle arrestors. The API Recommended Practice 1111 gives various recommendations and formulae which indicate above which depth the arrestors are recommended, necessary or strictly indispensable. Such devices were firstly proposed within the context of single-walled rigid pipes.
The arrestor may be movable, as described in the document U.S. Pat. No. 3,747,356. According to that document it is proposed to link a cylinder to a cable, to lodge the cylinder inside a pipe section and then to simultaneously unreel the pipe and the cable so as to keep the cylinder in the pipe section while the latter is being laid, until the pipe comes into contact with the seabed. The cylinder is then brought back up so as to be lodged in another pipe section to be laid, which is joined to the previous section. Consequently, any buckle likely to occur, when laying the pipe, between the pipelaying barge and the seabed is immediately arrested and is therefore not allowed to propagate along the pipe sections. However, such an arrangement provides no solution or effectiveness for arresting buckles likely to be propagated after the pipe has been finally laid on the seabed.
This is why fixed arrestors have been proposed, these being positioned in places along the pipe. The distance separating two arrestors is defined according to the cost of these devices compared with the cost of replacing the crushed section between two devices. In general, these devices are positioned along the rigid pipe every 100 to 300 m. These devices, by locally increasing the inertia of the pipe, make it possible to arrest the propagation of a buckle.
According to a first solution, an inner, or preferably outer, reinforcing collar (possibly in two parts, constituting a “clamp”) is used. Thus, in U.S. Pat. No. 3,768,269, it is proposed that the stiffness of the pipe be increased locally by placing, at regular intervals, reinforcing collars whose length ranges between 1 m and 2.5 m. Such a solution is valid only for pipes laid in sections since the reinforcing collars can be mounted and fastened to the pipe sections in the factory and then transported by the pipelaying barge to the laying site. When the pipe is long and wound onto a storage reel, it then becomes virtually impossible to wind the pipe with its reinforcing collars onto a reel since they would result in straight or almost straight portions that cannot be deformed when winding the pipe onto the storage reel. To mitigate this difficulty, it is conceivable to mount and fasten the reinforcing collars during the laying operations. However, it would then be necessary to interrupt the laying, at regular intervals, so as to mount and fasten the reinforcing collars. According to an alternative solution known through this same patent or through documents GB 1 383 527 or U.S. Pat. No. 5,458,441, the localized reinforcement may take the form of a thicker intermediate sleeve welded to the ends of the pipe.
According to a second solution, a spiralled rod is used on the external wall of the pipe. Thus, to allow the pipe to be wound onto a reel, U.S. Pat. No. 4,364,692 proposes to wind a rod tightly around the pipe so as to form a certain number of turns which can be welded at their ends to the rod itself and/or to the pipe.
According to another embodiment, the turns may be individual turns, by welding their two ends and by regularly spacing them apart along that portion of the pipe to be reinforced.
As long as the pipe is a single-walled pipe, the increase in the diameter in the reinforced portions may be acceptable. However, when the pipe is of the double-walled or pipe-in-pipe type, that is to say one comprising an outer tube or carrier pipe into which the inner tube or flowline is inserted, the increase in the diameter of the outer tube is unacceptable when transporting and storing long lengths of double-walled pipe when the pipe is laid by the “unreeled rigid pipe” method.
In addition, when the rigid pipe to be laid is manufactured in long lengths on land and then wound onto a reel on the pipelaying barge, the solutions recommended in the aforementioned documents are not appropriate as they use either long reinforcing collars, having a length of about 1 to 2.5 m, as in U.S. Pat. No. 3,768,209, or the winding of a reinforcing rod around the rigid pipe, as in U.S. Pat. No. 4,364,692.
In the case of double-walled pipes, the arrestor device generally consists of a collar mounted on the outer tube of the pipe. When the pipe is laid using a barge for laying an unreeled rigid pipe, this device must be mounted progressively with the laying operation (it is not reelable), thereby considerably increasing the cost of laying.
For the purpose of solving these problems and of obtaining double-walled pipes that can be wound, despite the propagation arrestors, the Applicant has already proposed particular devices.
According to Application FR 99/08540, a portion of flexible pipe or spring is welded to the internal wall of the outer tube in order locally to increase the inertia of the outer tube and form a flexible propagation arrestor which is reelable.
According to Application FR 99/15216, the propagation arrestor consists of an annular compartment filled with resin which is injected before or during the laying and which can be cured only after the laying if the length of the compartment for the resin is too great to allow winding in the cured state. The compartment serves to transfer the loads on the outer tube to the inner tube.
According to Application FR 00/00849, the arrestor consists of a steel partition which, as in the previous solution, transfers the loads from the outer tube to the inner tube.
All these buckle propagation arrestor devices therefore consist in reinforcing the outer tube, which may be subjected to this propagation, thereby directly or indirectly increasing the inertia of the outer tube.
The devices developed by the Applicant give very good results and are effective down to depths of the order of 2500 m. In some applications, when the depth is smaller, these devices may represent an excessive cost compared with the actual requirements.